1. Field of the Invention
The present invention relates to a magnet pump, and more particularly to a corrosion-resistant magnet pump comprising a front casing, a rear casing provided behind the front casing with a partition wall interposed therebetween, a rotary shaft extending from the front casing into the rear casing and supported by a bearing provided at least in the partition wall, an impeller fixed to the rotary shaft within the front casing, a driven magnet drivingly connected to the rotary shaft within the rear casing and a drive magnet provided outside the rear casing and drivingly rotatable by a motor, the portions of the pump to be exposed to a liquid being made of a corrosion-resistant material, the impeller being fitted around the front end of the rotary shaft and fastened to the rotary shaft by an impeller nut screwed on the front end.
The term "corrosion-resistant" as used herein refers to the property of being free from or not susceptible to attacking by acids, alkalis, salts, organic solvents, etc., or to resistance to chemicals. This means that the material concerned does not, or is unlikely to, deteriorate, crack, embrittle, discolor, permit penetration of some liquid thereinto or become otherwise degraded.
2. Description of the Prior Art
As shown in FIG. 19, the impeller Im of the corrosion-resistant magnet pump of the type mentioned above usually includes an insert In made of noncorrosive iron or like metal and positioned at the center of the impeller, and a corrosion-resistant synthetic resin portion W (including blades) provided around and formed integrally with the insert. The insert In is fitted around the forward end of a rotary shaft Sh made also of a noncorrosive material and is fastened to the shaft by an impeller nut N of corrosion-resistant synthetic resin in the form of a cap nut and screwed on the shaft end. The rotary shaft Sh is covered with corrosion-resistant synthetic resin at least over a stepped portion a thereof in contact with the impeller Im and over a portion b extending from the portion a. To protect the shaft Sh and the insert In, an O-ring or like seal means Se is interposed between the impeller Im and the impeller nut N, as well as between the impeller Im and the synthetic resin layer over the stepped portion a in contact therewith. Furthermore, a thrust bearing Sb for withstanding the thrust f which is likely to act on the impeller rearwardly of the pump is interposed between the impeller Im and the partition wall (not shown in FIG. 19) between the unillustrated front and the rear casings, or between the rotary shaft and the rear casing (not shown in FIG. 19).
Nevertheless, even if the impeller nut N is tightened up with the use of the O-ring or like seal means S, the impeller nut N will loosen owing to the vibration of the pump, intentional reverse rotation of the pump or the reverse rotation of the pump due to a reverse flow from the discharge pipe toward the pump upon stopping of the pump. Aside from this phenomenon, the seal means will deteriorate during the operation of the pump. The pump therefore has the problem that the liquid handled will attack the rotary shaft S or the impeller insert In if it is corrosive. Moreover, the conventional pump is complex in the construction of the impeller Im and the arrangement around the impeller.